The present invention relates to a method and a device for phase-shifting a signal that has been generated by an oscillator which is controlled by a reference oscillator via a phase locked loop.
Oscillator devices for generation of different frequencies often consist of a voltage controlled oscillator (VCO) whose output signal is locked by means of a phase locked loop to a reference signal. For attaining a good stability, among other things, the reference signal is generated by a high quality, mostly crystal controlled, voltage controlled oscillator (VCXO). Such oscillator devices have many uses, for example as local Oscillators for radio communication and radar equipment.
Normally, there is a wish to be able to generate different frequencies from the same oscillator device. It is relatively simple to lock the frequency of the voltage controlled oscillator to an integer multiple of the frequency of the reference signal by means of the phase locked loop. To only be able to generate integer multiples of the frequency of the reference signal is, however, a limitation that frequently cannot be accepted.
To make it possible to choose frequency more freely, frequency dividers have for example been introduced for both the reference signal as well as for the output signal of the controlled oscillator before it is compared in the phase locked loop. By choosing different divisors for the two signals, it is thereby possible to considerably increase the possibilities to choose frequency.
Another way to increase the flexibility of adjustment of the frequency is described in U.S. Pat. No. 4,668,923. According to the described device in the patent, a reference signal is periodically phase rotated before it is compared with the output signal of the controlled oscillator. By choosing different values of the size of the phase rotation, it is in principle possible to tune to any frequency that is higher than the reference signal.
The device according to the US patent thus provides good possibilities for frequency adjustment. In certain applications, it is however desirable to be able to also adjust the phase of the signal. This is however not possible with the device described in the U.S. patent and nor is it possible in the devices mentioned above where the frequency is set by a selection of different divisors.